In the manufacture of products using thermoplastic resins such as polyolefins, various additives are generally included in the resin to affect color, to ease processability, and to inhibit oxidation and other types of degradation, to stabilize the physical characteristics of the resin and thus prolong the life expectancy of the product.
For maximum effectiveness, it is important that any additive be uniformly distributed in the thermoplastic resin. Poorly distributed additives may contribute to unsatisfactory properties in the final product, such as reduced tensile and tear strengths, reduced resistance to low temperature flexing, reduced elongation characteristics, reduced breakdown voltage strengths of dielectric materials, and electrical losses caused by an increased power factor and increased dielectric constant.
The physical form of plastics additives can determine the efficiency and economics of their introduction into the thermoplastic material. For example, fine powders tend to be fairly readily dispersed but are difficult to handle and can cause environmental problems. They are also difficult to introduce continuously into process equipment.
In practice, a number of procedures have been employed to incorporate solid additives into polymeric systems. Conventional additive delivery systems use dry additive feeding and mixing with polymer resins, in which dry plastic additives are metered and mixed with polymer particles in blenders or mixers. Alternatively, dry additives are mixed with a resin as it is processed through a pelletizer, extruder, or compounding device. In another process, the additives are melted and coated on plastic resin particles before introducing them into an extruder. High melting additives are difficult to control in this technology. In yet another process, resin particles are coated with an aqueous emulsion of the additives, then dried. This procedure is not suitable for hydrolyzable additives such as many phosphite antioxidants, and the water must ultimately be removed, resulting in complexity and expense. Other methods involve, for instance, dissolving additives in one or more of the components of the mixture to be polymerized before the polymer is formed, or mixing the additive in a solution, suspension, or emulsion of the polymer and then removing the solvent or suspending agent.
The literature also contains descriptions of spray procedures for introducing plastics additives into polymeric materials, and for spraying various other materials in supercritical carbon dioxide. A number of these references are discussed briefly below.
U.S. Pat. No. 5,007,961 and corresponding PCT application WO 90/02770 disclose aqueous systems for applying additives to polymeric particles, as well as methods for applying such additive systems such as spraying, wiping, or dipping, and polymeric particles treated with such additive systems. The additive systems comprise an emulsified wax, surfactant, base, one or more functional polymer additives, and water.
European patent application 411,628 discloses stabilizing polyolefins in non-extruded as-polymerized particle form by depositing on the particles a mixture of stabilizers including one or more organic phosphites or phosphonites and one or more phenolic antioxidants. Optional ingredients are thioethers, organic polysulfides, hindered amine light stabilizers, benzophenone and benzotriazole derivatives, and diluents such as paraffins, cycloparaffins, epoxidized soybean or linseed oil, silicone oils, and olefin oligomers. The stabilizer mixtures are applied, in a melted state or in a liquid state by virtue of containing liquid phosphites or phosphonites, by a continuous or batch mixer optionally equipped with a spraying mechanism.
U.S. Pat. No. 5,041,310 of Williams discloses a coating composition comprising a mixture of polymer additives, gelling agent, and oil, which is applied as a liquid to the surface of particles of polymer, and caused to gel.
U.S. Pat. No. 4,960,617 discloses a process for post-reactor stabilization of polyolefins by melting a polyolefin wax, blending at least one additive into the resulting melt, fluidizing polyolefin particles to be stabilized with hot gas, and spraying the liquid polyolefin wax containing at least one additive on the fluidized polyolefin particles.
U.S. Pat. No. 4,882,107 discloses a method and apparatus for spraying a solution, suspension, or dispersion of a mold release material in a supercritical fluid such as supercritical carbon dioxide onto the surface of a mold, to coat it with the release agent.
U.S. Pat. Nos. 4,923,720 and 5,027,742 and Chemical Abstract 113:154288p disclose a process and apparatus in which supercritical fluids such as supercritical carbon dioxide are used to reduce the viscosities of viscous coating compositions to permit their application as liquid sprays.
U.S. Pat. No. 5,066,522 discloses the use of supercritical fluids such as supercritical carbon dioxide as diluents in liquid spray applications of adhesives.
European patent application 350,910 discloses liquid spray application of coatings with supercritical fluids as diluents, and spraying from an orifice.
Production of fine powders in inorganic oxides and certain drugs by rapid expansion of supercritical fluid solutions has been reported. See Chemical Abstracts 108:155263k, 105:197085x, 105:63102s, and 104:227104b. Graphite has also been produced in a micro-powder form by wetting it with liquid CO.sub.2 then vaporizing the CO.sub.2 at a temperature and pressure above the critical point of CO.sub.2 gas. See Japanese patent publication 62/265111. However, it does not appear that particle size reduction of plastics additives in nonvolatile liquid matrices upon spraying in supercritical CO.sub.2 has been reported.
Despite the progress made in applying polymer additives to polymers in spray processes, indicated by the references discussed above, prior art processes generally suffer from certain deficiencies. Some liquid systems have high viscosities which make them difficult to atomize without heating, dilution, use of a high amount of atomization gas, and/or use of relatively high pressures for spraying. Systems which involve the spraying of materials which are solids under standard conditions can experience difficulties related to handling or melting of the solids, and plugging of lines as a result of resolidification of the solid materials in vessels, piping, and the spray nozzle. Such operational difficulties can make the spraying operation inefficient, adversely affecting not only its economics, but also the control of the amounts of the additives and the uniformity of their application to the polymer being treated. An improved spray process for applying plastics additives to polymers would be very desirable. Such a process is the subject of this application.